Who made it?

Very few examples of salt glazed stoneware are marked before 1870, unless you are lucky enough to possess items by Briddon (Chesterfield) or Kishere (Mortlake).

The most useful guide to identification of an unmarked item is the sprig under the handle (with the exception of Doulton these are rarely copied).

Below are the 5 most readily identifiable makers in alphabetical order.

Denby
  Vine leaf variants 1850s on.
   
 
  1840s?
   
Doulton

 

Most small sizes, 1870 on Small sizes, 1850s-60s? Most medium & large sizes
   
Fulham  
  Standard for all sizes  
   
Smith  
  Standard for all sizes  
   
Stiff

 

Small Medium Large
 

Note

Doulton terminal sprigs were copied by Port Dundas (Glasgow) and Fulham but usually identified with making pottery's name.
Stiff terminal sprigs were copied by Doulton for their "old style range" but again marked with maker's name.
Smith copied Doulton and Stiff body sprigs but retained their usual handle terminal.

Although this covers probably 80% of all jugs (and tankards with handle terminals) you may well see items with terminals like these:

  

The left terminal is discussed under Unidentified maker 1 and the right is called, rather unhelpfully, in reference books "Standard English Terminal" or SET.
Leafy terminals exist in many variations but were definitely used by Bristol (Price and Powell), Kishere (Mortlake) and Vauxhall.